Microstrip transmission lines provide low-loss transmission of microwave and millimeter wave signals, particularly over extended distances as for example in antenna transmission lines. Coplanar wave guides are useful in microwave and millimeter wave circuits for transmitting microwave signals over one face of a substrate. They are ideally suited, for example, for flip chip mounted amplifiers.
Conversion between microstrip transmission mode and coplanar wave guide mode offers the circuit designer the advantages of low loss transmission of microstrip with extra-low common lead inductance associated with amplifiers, oscillators, or other circuits having active devices flip-chip mounted onto coplanar wave guides. Low common lead inductance is very important in amplifier design in particular principally because the inductance common to the input and output circuit reduces the stable gain of the amplifier.
Connecting a microstrip transmission line to a coplanar wave guide in the usual, passive way introduces loss due to the fact that the conversion usually takes several quarter wavelengths of transmission line to achieve. It is desirable to "launch" a microwave signal input supplied in a microstrip transmission line mode to a coplanar wave guide transmission mode with no loss of gain and have the Signal leave the coplanar wave guide with no loss in gain. It would be further desirable if such connection could be in the form of a monolithic microwave integrated circuit (MMIC), occupying a small space with few parts and therefore costing less to produce than current connection methods.